This invention relates in general to noise responsive devices and in particular to an electronic circuit for automatically adjusting the sensitivity to a microphone in response to the level of ambient noise.
Microphones are well known devices for converting acoustic energy into electrical energy. Microphones are widely used as audio input devices for many different types of electrical and electronic devices, such as aviation radios, vehicle telephones, audio and video tape recorders, and other communication and sound recording devices. A wide variety of microphone structures are known in the art for use with such devices.
Typically, a given microphone structure has a predetermined sensitivity characteristic. Generally speaking, this sensitivity characteristic is representative of the magnitude of the output electrical signal generated by the microphone in response to the input acoustic energy sensed thereby. Thus, a high sensitivity microphone can generate an electrical signal in response to relatively small amounts of acoustic energy. High sensitivity microphones are frequently used in situations where the undesirable ambient noise underlying a desired source is relatively small, such as in studio sound recordings. Conversely, a low sensitivity microphone can generate an electrical signal in response only to relatively high amounts of acoustic energy. Low sensitivity microphones are frequently used in situations where the undesirable ambient noise underlying a desired source is relatively large, such as in aviation radios.
In some situations, a microphone structure may be used in conjunction with an electronic switch which functions as a threshold adjustment circuit. The threshold switch can be controlled between an open mode (wherein the signal from the microphone is not passed therethrough) and a closed mode (wherein the signal from the microphone is passed therethrough). Such threshold switches are frequently provided where the microphone is used in an environment having a relatively high amount of ambient noise. In use, the threshold switch is normally operated in the opened mode until a threshold level of acoustic energy (typically somewhat higher than the level of the ambient noise) is sensed. At that point, the threshold switch is operated in the closed mode, allowing the signal to be passed therethrough.
As mentioned above, a given microphone structure has a predetermined sensitivity characteristic. Therefore, a microphone which is designed for use in a certain situation may not be suitable for use in a different situation. Furthermore, some microphones may be used in situations where the undesirable ambient noise underlying a desired source changes during use. For example, the level of ambient noise underlying the use of a vehicle telephone may change dramatically, depending upon the speed of the car, whether the windows of the vehicle are open or closed, and many other factors. Although manual and automatic microphone sensitivity control circuits are known in the art, such circuits are typically complicated and expensive. Similarly, manual and automatic microphone threshold control circuits are also typically complicated and expensive. Thus, it would be desirable to provide an automatic microphone sensitivity control circuit and an automatic microphone threshold control circuit which are simple and inexpensive in construction and operation.